This patent application claims priority based on a Japanese patent application, H10-149227 filed on May 29, 1998, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a radio receiver which receives a signal modulated by angle modulation, and in particular to a radio receiver which switches the received signal based upon a supervisory signal indicative of the condition of the propagation path through which the signal passes.
2. Description of the Related Art
Radio receivers employing angle-modulation have been widely used. In contrast to amplitude modulation, angle-modulation utilizes a wide band to improve the signal-to-noise ratio. However, this improvement has a limit, which in general is called the threshold. If the reception level of the audio signal is decreased to this threshold, the noise accompanying the audio signal sharply increases. In this case, it is necessary to cut off the audio signal, and a supervisory signal is used to indicate whether or not the audio signal should be cut off.
For example, the audio signal is cut off when the supervisory signal indicates that the propagation path is bad, so as to avoid providing an audio signal having a lot of noise to the user of the radio receiver.
FIG. 1 shows the configuration of a conventional radio receiver. The radio receiver receives from a radio transmitter (not shown) an audio signal and a supervisory signal. For example, as the supervisory signal, the AMPS (Advanced Mobile Phone Service) system employs a SAT (Supervisory Audio Tone) signal such as a 5970 Hz signal, a 6000 Hz signal, and a 6030 Hz signal. In FIG. 1, to control the audio signal as instructed by the supervisory signal, the radio receiver incorporates an antenna 1000, a demodulator 1001, a high pass filter (HPF) 1002, an audio demodulator 1003, a controller 1004, and a speaker 1005. Further, the audio demodulator 1003 incorporates a band pass filter (BPF) 1003a, a de-emphasizer 1003b, and an expander 1003c. The controller 1004 incorporates a detector 1004a and a switch 1004b, wherein the detector 1004a incorporates a band pass filter 1004c, a level detector 1004d, and a switch controller 1004e as shown in FIG. 2.
Upon receipt of an audio signal and a supervisory signal by the antenna 1000, the demodulator 1001 demodulates those signals, whereby the demodulated signals are fed into both the high pass filter 1002 and the audio demodulator 1003. Since the high pass filter 1002 permits signals having frequencies higher than the frequency of the audio signal to pass therethrough, the supervisory signal passes through the high pass filter 1002, thus entering the detector 1004a of the controller 1004. Since the band pass filter 1003a allows the audio signal to pass therethrough, the audio signal enters the switch 1004b of the controller 1004. Upon receiving the supervisory signal, the detector 1004a detects whether the level of the supervisory signal is higher or lower than a given threshold, so as to control the switch 1004b. More specifically, the band pass filter 1004c passes only the supervisory signal, the level detector 1004d detects the level of the supervisory signal, and the switch controller 1004e controls the switch 1004b according to the detected level. If the supervisory signal is higher than the threshold, the detector 1004a instructs the switch 1004b to pass the audio signal to the speaker 1005. On the contrary, if lower, the detector 1004a instructs the switch 1004b to cut off the audio signal. In this way, if the supervisory signal indicates that the path condition is good, that is, that the audio signal contains little noise, the audio signal is provided to the user; however, if the supervisory signal indicates that the path condition is bad, that is, that the audio signal contains a lot of noise, the audio signal is not provided to the user, so as not to annoy the user.
However, the detector 1004a detects the level of noise in addition to the level of the supervisory signal. The level of noise might be higher than the level of the supervisory signal owing to fading, for example. Accordingly, even though the level of the supervisory signal is low, a high level of noise might, by acting like the supervisory signal, allow the audio signal to enter the speaker 1005. This would provide the user with an audio signal containing a lot of noise, thus annoying the user.
On the other hand, the Japanese national publication of the translated version of Hei 1-500788 teaches another radio receiver. This radio receiver controls or reduces the level of the audio signal by comparing a given threshold with the amplitude envelope of the received signals which are beyond the frequency band of the audio signal. In addition, a technique of reducing the audio signal according to an RSSI (Received Signal Strength Indicator) signal indicating the level or strength of the received signal has been used. However, neither the former technique nor the latter technique has solved the problem discussed above.
It is therefore an object of the present invention to provide a receiver which is capable of correctly controlling the outputting of an audio signal according to a supervisory signal regardless of the level of the noise accompanying the supervisory signal.
According to an aspect of the present invention, there is provided a receiver, which receives a desired signal, comprising: a calculating circuit which calculates the condition of the desired signal based upon the level of the desired signal and the level of noise accompanying the desired signal; and a controlling circuit which controls the outputting of the desired signal according to the calculated condition.
According to another aspect of the present invention, there is provided a portable telephone which receives a voice signal modulated by angle modulation and a supervisory signal for checking the condition of the propagation path through which the voice signal passes, and controls the outputting of the voice signal according to the supervisory signal, the portable telephone comprising: a receiving circuit which receives a radio wave including the voice signal and the supervisory signal; a first filter which passes the supervisory signal; a second filter which passes noise whose frequency is different from the frequency band of the voice signal and the frequency of the supervisory signal; a first level calculator which calculates the level of the supervisory signal passing through the first filter; a second level calculator which calculates the level of the noise passing through the second filter; a subtractor which calculates the difference between the level of the supervisory signal and the level of the noise; a comparator which compares the difference with a threshold defined by the angle modulation; and a controller which controls the outputting of the voice signal according to the result of the comparison obtained by the comparator.
According to still another aspect of the present invention, there is provided a method for use of a radio receiver which receives a transmission signal and a supervisory signal for supervising the condition of the propagation path through which the transmission signal passes, and controls the outputting of the transmission signal according to the supervisory signal, the method comprising the steps of: detecting the level of the supervisory signal; detecting the level of noise whose frequency is different from the frequency band of the transmission signal and the frequency of the supervisory signal; and calculating the difference between the level of the supervisory signal and the level of noise; and controlling the outputting of the transmission signal according to the difference.